1. Field of the Invention
The present invention relates generally to electrochemical analysis apparatus and, more particularly, to an analyzer capable of automatically and directly analyzing biological and biochemical substances found, for example, in blood serum.
2. Description of the Prior Art
A variety of techniques and apparatus have been developed in the past for analyzing small blood samples for the presence of bicarbonate, chloride, and other constituents. The use of these measurements is well known to physicians and other clinical and diagnostic specialists for applications in surgery, pulmonary function studies, cardiac catheterization, anesthesia, as well as in other related fields.
Serum bicarbonate, which is present in human blood at about 30 milliequivalents per liter, has been measured by a variety of methods all of which involve acidifying the blood sample to release carbon dioxide. These methods include back titration with a base of the remaining excess of a known amount of acid added to a blood sample; colorimetric measurement of the carbon dioxide which diffuses from the sample solution across a dialysis membrane into a basic solution containing an acid-base indicator; colorimetric measurement of the carbon dioxide swept out of the sample solution into a basic scrubbing solution containing an acid-base indicator; gasometric measurement of the volume of carbon dioxide released from the sample; and calculation of the carbon dioxide in the sample using a conventional pCO.sub.2 electrode to measure the partial pressure of carbon dioxide and a pH electrode to measure the pH of the sample.
While the above methods generally will provide some measure of bicarbonate, they have not proven satisfactory in many respects. Back titration with base of the acid excess in an indirect measurement technique performed after the carbon dioxide has been released to the atompshere. Being indirect, the method is less accurate than methods employing direct measurement of the carbon dioxide actually released. Moreover, titration must proceed to its end point before a measurement is available.
Colorimetric measurements of carbon dioxide released from the sample are time consuming and complex. It is difficult to sweep out and to adsorb the carbon dioxide released. The reactions involved must proceed to equilibrium over a prolonged period, and thus such methods are unsatisfactory for providing rapid and reliable sample measurements.
Gasometric measurement of the carbon dioxide released is also slow and, moreover, requires highly trained operators for proper execution.
Conventional pCO.sub.2 electrodes are inherently slow to equilibrate after exposure to carbon dioxide, and therefore such electrodes are generally unsuited to the automatic and rapid analysis of samples. The equilibration process must generally be permitted to occur twice for each measurement, once with the CO.sub.2 containing sample, and once again with the fresh reagent prior to sample introduction.
In view of the foregoing, it is apparent that a need exists for analysis apparatus for performing blood chemistry analysis and the like which can provide rapid and accurate measurements with minimal operator intervention and with a minimum level of required operator skill. The apparatus should overcome the disadvantages of the bicarbonate measuring methods enumerated above and should preferably do so in a manner which further enables simultaneous measurement of other constitutents, such as chloride, of the same sample.